(1) Field of the Invention
The present invention relates to a routing system for a linear add-drop multiplexer, and more particularly to a routing system for a linear add-drop multiplexer having a cross-connect function.
(2) Description of the Related Art
Recent demands for higher performance optical transmission systems require add-drop multiplexers (ADM) with a cross-connect function. Generally, ADMs are classified into linear ADMs for use in a linear transmission line and ring ADMs for use in a ring-shaped transmission line. Linear ADMs receive signals from terminals connected to the opposite ends of a bidirectional linear transmission line, and transmit signals to the terminals. When a linear ADM is to receive a signal, one of the terminals (hereinafter referred to as East and West terminals) is selected for a path in use and the other for a path not in use. A channel connected to the terminal selected for a channel in use, is dropped. To drop the channel, the ADM has a routing unit for establishing the path in use to drop the channel to lower signal levels. For signal transmission, one of the East and West terminals is selected for a path in use and the other for a path not in use, with an add channel connected to the path in use. To connect the add channel, the ADM has a gate for transmitting a signal to the path in use.
FIG. 1 of the accompanying drawings is a diagram of an overall arrangement of an optical transmission system in which ADMs are disposed between linear transmission lines. As shown in FIG. 1, ADMs 72, 73, 74 are disposed between linear optical transmission lines 70a through 70h, with line terminal equipment (LTE) 71, 75 connected to opposite ends thereof. A multiplexed optical signal is transmitted at a rate of 600 Mbits/s over the optical transmission lines 70a through 70h. To the LTE 71, 75 and the ADMs 72, 73, 74, there are connected transmitter/receivers 76, 77, 78, 79 and 80 for lower signal levels, which are connected to exchanges 81, 82, 83, 84, 85, 86, 87, 88 and 89. Telephone sets 90, 91, 92, 93, 94 and 95 or other terminal devices are connected to the exchanges 81, 82, 83, 84, 85, 86, 87, 88 and 89. Input/output devices 96, 97 and 98 are connected to the respective ADMs 72, 73, 74 for applying path connecting commands to the ADMs 72, 73, 74.
In operation, multiplexed optical signals transmitted from the LTE 71, 75 are converted into electric signals and then demultiplexed by the ADMs 72, 73, 74. The ADMs 72, 73, 74 then transmit lower-level signals to the transmitter/receivers 76, 77, 78, 79 and 80 through a path that has been established, thereby providing a drop channel. The ADMs 72, 73, 74 pick up signals, through a path that has been established, out of the lower-level signals transmitted from the corresponding transmitter/receivers 76, 77, 78, 79 and 80, and also out of demultiplexed signals (through signals) that are not transmitted to the transmitter/receivers 76, 77, 78, 79 and 80, thus establishing an add channel. Then, the ADMs 72, 73 and 74 multiplex the picked-up signals, convert them into optical signals, and output the optical signals to the LTE 71, 75. The LTE 71, 75 also establish a drop channel or an add channel based on the established path. The transmitter/receivers 76, 77, 78, 79 and 80 convert the signals that have been transmitted at a rate of 45 Mbits/s from the LTE 71, the ADMs 72, 73, 74 and the LTE 75, into signals at a rate of 1.5 Mbits/s, and transmit the converted signals to the exchanges 81 through 89. Alternatively, the transmitter/receivers 76 through 80 convert signals transmitted at 1.5 Mbits/s from the exchanges 81, 82, 83, 84, 85, 86, 87, 88 and 89 into signals at 45 Mbits/s, and transmit the converted signals to the LTE 71, the ADMs 72, 73, 74 and the LTE 75. The exchanges 81, 82, 83, 84, 85, 86, 87, 88 and 89 convert the signals that have been transmitted at 1.5 Mbit/s from the transmitter/receivers 76, 77, 78, 79 and 80, into signals at a rate of 64 Kbits/s, and transmit the converted signals to the telephone sets 90, 91, 92, 93, 94 and 95. Alternatively, the exchanges 81, 82, 83, 84, 85, 86, 87, 88 and 89 convert signals at 64 Kbits/s from the telephone sets 90, 91, 92, 93, 94 and 95 into signals at 1.5 Mbits/s, and transmit the converted signals to the transmitter/receivers 76, 77, 78, 79 and 80.
FIG. 2 of the accompanying drawings is a block diagram of a conventional ADM for use in the above optical transmission system. A multiplexed optical signal of higher signal level transmitted from an optical transmission line 111 connected to an East terminal is converted into an electric signal by an optoelectronic (O/E) transducer 112. The electric signal is then separated by a demultiplexer (DMUX) 113 into a plurality of channels, which are then transmitted to drop switches 114 and add switches 134. There are as many drop switches 114 and as many add switches 134 as the number of channels. Each of the drop switches 114 is connected to all of the channels, and each add switch 134 is connected to one of the channels, thus establishing a through channel. The drop switches 114 select, as a drop channel, one of the channels separated by the demultiplexer 113 in response to a selection signal from a controller 160, and transmit the signal through the selected channel to channel switches 150 for lower signal levels. There are as many channel switches 150 as the number of channels, and the signals from the drop switches 114 are transmitted to these channel switches 150. A drop channel to be selected has previously been indicated to the controller 160 by keystrokes entered through an input/output device 165. The selection signal applied to the drop switches 114 represents the drop channel thus selected.
The channel switches 150 select either the output signal from the drop switches 114 (i.e., the signal received from the East terminal), or an output signal from drop switches 144 (described below--i.e., a signal received from a West terminal), and transmit the selected signal to a transmitter/receiver 170 for lower signal levels. The above selection made by the channel switches 150 is based on a selection signal that is produced by the controller 160 according to keystrokes entered through the input/output device 165, and which is applied to path switches 150a in the respective channel switches 150.
Similarly, a multiplexed optical signal of higher signal level transmitted from an optical transmission line 141 connected to a West terminal is converted into an electric signal by an optoelectronic (O/E) transducer 142. The electric signal is then separated by a demultiplexer (DMUX) 143 into a plurality of channels, which are then transmitted to drop switches 144 and add switches 124. There are as many drop switches 144 and as many add switches 124 as the number of channels. Each of the drop switches 144 is connected to all the channels, and each add switch 124 is connected to one of the channels. The drop switches 144 select, as a drop channel, one of the channels separated by the demultiplexer 143 in response to a selection signal from a controller 160, and transmit the signal through the selected channel to the channel switches 150 for lower signal levels. As described above, there are as many channel switches 150 as the number of channels, and the signals from the drop switches 144 are transmitted to these channel switches 150.
A signal transmitted from the transmitter/receiver 170 is sent through gates (not shown) in the channel switches 150 to the add switches 124 and the add switches 134. The channel switches 150, which are as many as the channels, are connected to the add switches 124, 134, so that all signals from the channel switches 150 are sent to any one of the add switches 124, 134.
In response to a selection signal from the controller 160, the add switches 124 connected to the East terminal select one of the signals in the channels and the one-channel signal from the demultiplexer 143, thus establishing an add channel, and outputs the selected signal to a multiplexer 123. The add switches 124 select the signal based on a selection signal transmitted from the controller 160 representing a channel to be selected, as indicated by the input/output device 165.
Inasmuch as there are as many add switches 124 as the number of channels, the multiplexer 123 is supplied with signals transmitted from the add switches 124 in the respective channels. The multiplexer 123 multiplexes the supplied signals signals into an electric signal, which is then applied to an electro-optic (E/O) transducer 122. The electric signal is then converted by the electro-optic transducer 122 into an optical signal that is outputted to an optical transmission line 121.
Likewise, responsive to a selection signal from the controller 160, the add switches 134 connected to the West terminal select one of the signals in the channels and the one-channel signal from the demultiplexer 113, and output the selected signal to a multiplexer 133. The multiplexer 133 multiplexes the signals supplied from the add switches 134 in the respective channels into an electric signal, which is then applied to an electro-optic (E/O) transducer 132. The electric signal is then converted by the electro-optic transducer 132 into an optical signal that is outputted to an optical transmission line 131.
In the linear ADM, the drop switches connected to the East terminal, the drop switches connected to the West terminal, the add switches connected to the East terminal, the add switches connected to the West terminal, and the channel switches for lower signal levels operate to change channels based on established paths. Therefore, the transmitter/receivers for lower signal levels can be connected to any channels connected to the East and West terminals. The above path exchange capability is known as a cross-connect function.
However, the input/output device 165 is required to command the controller 160 for establishing paths with respect to the five switch assemblies, i.e., the drop switches 114, 144, the add switches 124, 134, and the channel switches 150 which are provided for each of a plurality of channels. Since the number of switch assemblies to be addressed for establishing paths is equal to 5 multiplied by the number of channels, the process of establishing paths with the input/output device 165 is complex and time-consuming. Therefore, there has been a demand for a simplified process of establishing paths which is to be carried out by the input/output device 165.
Heretofore, the path switches 150a in the respective channel switches 150 operate to determine which paths connected to the East or West terminal, are to be dropped based on the selection from the controller 160. The channel switches 150 are normally composed of respective individual block units. Consequently, the block units of the channel switches 150 are connected to the controller 160 by respective control lines. A program for establishing paths with respect to the channel switches 150 approaches the individual path switches 150a in the channel switches.
The drop switches connected to the East terminal, the drop switches connected to the West terminal, the add switches connected to the East terminal, and the add switches connected to the West terminal are normally assembled as four block units, respectively. These four block units are connected to the controller 160 through respective control lines. Therefore, no individual control lines are connected from the controller 160 to the individual drop or add switches. A program for establishing paths with respect to these switches first approaches the block units, and then controls the individual drop or add switches.
The program for establishing paths with respect to the channel switches is more complex than the program for establishing paths with respect to the drop and add switches, because the former program is required to approach the individual channel switches rather than a single block unit. As a consequence, the program for establishing paths with respect to the channel switches is relatively large in scale and highly expensive to develop.